1. Field of the Invention
This invention generally relates to an array antenna system for directing and receiving electromagnetic energy and more particularly to a fiber optic true time-delay array antenna feed system.
2. Description of the Related Art
An antenna array is a stationary group of individual radiating and collecting elements that collectively form electronically steerable transmit and receive beams. The electromagnetic radiation signal driving and received by each element is identical to or from each other element except for the relative time delays present. To achieve a desired steering of the electromagnetic beam a technique must be included in the architecture of the system that provides a variable time delay scheme to allow the electromagnetic signal to be processed by the different antenna elements at varying times. In some systems this is achieved by routing numerous conventional coaxial or waveguide transmission lines of varying lengths, however this system is unwieldy, heavy, expensive and lossy.
Other techniques, such as the use of optical fiber transmission lines, have been used. To achieve the time delay required in the array antenna systems the technique of electromechanical optical fiber stretching has been proposed. See, U.S. Pat. No. 4,814,774, Herczfeld (1989). Here time delays are introduced into the signal delivered by each antenna element by stretching the fiber-optic link feeding each element. However, to produce significant time delays (.about.1 ns), long fiber lengths are required, thereby resulting in large stretching forces.
Various different switching schemes involving switching in and out or selecting various different discrete fiber-optic delays have been proposed. A wavelength dependent, tunable, optical delay system based on selectively varying the wavelength of optical light is described in U.S. Pat. No. 4,671,604, Soref. Utilizing plurality of wavelength filter combinations or cleaved-coupled-cavity (C.sup.3) lasers, the time-delay of the signal exiting the fiber can be varied over a preselected range by selectively varying the wavelength of the optical carrier signal. However, to effectively and accurately delay the optical signal, the output of all lasers and the wavelength of all tunable filters must be changed equally and simultaneously. The device in Soref is, thus, complicated to operate and has many expensive components. Further, Soref specifies direct radio frequency (RF) modulation of the optical source, which results in an interaction between the RF signal and the optical carrier signal.